Briton Jenson Button [ Images ], starting on pole position for the first time on a poignant weekend commemorating Ayrton Senna's death at Imola 10 years ago, raced to a career best second place for BAR.
Colombian Juan Pablo Montoya [ Images ] was third in a Williams.
Schumacher had previously won the first four races of the season for Benetton in 1994.
He leads the drivers' championship with 40 points, Barrichello has 24 and Button 23. Ferrari lead the constructors' standings with 64, ahead of Renault [ Images ] on 31 and BAR and Williams level on 27.
The win was Schumacher's fifth in six years at Imola, Ferrari's home track that is possibly hosting the grand prix for the last time, and the sixth in total after his first in that fateful 1994 race.
The German now has a record 74 career wins and was in implacable form, crossing the line with an advantage of 9.7 seconds to maintain the Schumacher family stranglehold on the race. Schumacher's younger brother Ralf won in 2001.
Ralf was seventh in a Williams after colliding with Alonso and spinning off 11 laps from the end. McLaren's [ Images ] young Finn Kimi Raikkonen [ Images ], who started from the back of the grid and was lapped by Schumacher, collected the final point.
It was the first finish of the season for last year's championship runner-up after repeated engine failures.